class Solution {
    boolean [][] vis;
    int [] dx={0,0,1,-1};
    int [] dy={1,-1,0,0};
    int m;
    int n;
    public int numIslands(char[][] grid) {
        m=grid.length;
        n=grid[0].length;
        vis=new boolean[m][n];
        int ret=0;
        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                if(!vis[i][j]&&grid[i][j]=='1'){
                    ret++;
                    bfs(grid,i,j);
                }
            }
        }
        return ret; 
    }
    public void bfs(char[][] grid,int i,int j){
        Queue<int[]> q=new LinkedList<>();
        q.add(new int[]{i,j});
        vis[i][j]=true;
        while(!q.isEmpty()){
            int[] t=q.poll();
            int a=t[0],b=t[1];
            for(int k=0;k<4;k++){
                int x=a+dx[k];
                int y=b+dy[k];
                if(x>=0&&x<m&&y>=0&&y<n&&!vis[x][y]&&grid[x][y]=='1'){
                    vis[x][y]=true;
                    q.add(new int[]{x,y});
                }
            }

        }
    } 
}